Agglomerated tripolyphosphate



United States Patent 3,387,923 AGGLOMERATED TRIPOLYPHOSPHA'I'E Chung Yu Shen, St. Louis, Mo., assignor to Monsanto Company, St. Louis, Mo., a corporation of Bel-aware No Drawing. Filed Dec. 30, 1964, Ser. No. 422,3fi4 14 Claims. (Cl. 23-106) ABSTRACT OF THE DTSCLOSURE A process for agglomerating finely divided particles of alkali metal tripolyphosphates which comprises heating the particles in an atmosphere having a temperature sufficient to render the surfaces of the particles to a partially liquid state but below the transition temperature at which all of the alkali metal tripolyphosphate at the surfaces of the particles is converted to alkali metal pyrophosphate, agitating the particles to thereby produce agglomerated particles of alkali metal tripolyphosphates and cooling the agglomerated particles to a temperature at which the surfaces are solidified before the nuclei of the particles become partially liquefied.

This invention relates to the production of alkali metal tripolyphosphates. More particularly, it relates to the agglomeration of finely divided alkali metal tripolyphates.

Alkali metal tripolyphosphates are produced by a wide variety of processes well known to those skilled in the art. These processes produce finely divided particles of tripolyphosphate. Dust created by the handling of these finely divided particles causes housekeeping and health problems. The problems generally necessitate the separation of the finely divided particles from the larger particles. The larger particles, having granular characteristics, are relatively dust free and generally are pre ferred. The finely divided particles having powdery characteristics frequently are stored and then sold to customers who are willing to handle the powdery material. Since the relatively dust free granular product is generally preferred by most customers, large inventories of the powdery material become serious problems.

By employing this invention, a producer of alkali metal tripolyphosphate can convert the finely divided alkali metal tripolyphosphate into the more desirable, relatively dust free form and eliminate the problems associated with the finely divided particles.

It is an object of this invention to provide a process for producing an agglomerated form of alkali metal tripolyphosphate from finely divided particles of alkali metal tripolyphosphate.

It is another object of this invention to provide a recycle process for converting finely divided particles of alkali metal tripolyphosphate to an agglomerated form of alkali metal tripolyphosphate.

It is a further object of this invention to provide a continuous process for converting finely divided particles of alkali metal tripolyphosphate to an agglomerated form of alkali metal tripolyphosphate.

These and other objects will become readily apparent to one skilled in the art from the detailed description following.

It has been discovered that finely divided particles of alkali metal tripolyphosphate, heated under proper temperature conditions, agitated and properly cooled will form relatively dust free, agglomerated alkali metal tripolyphosphate.

In the practice of the present invention, it is necessary to heat the finely divided particles to form viscidsurfaced particles. Viscid-surfaced particles, as used herein, are particles, the surfaces of which are below the transition temperature at which all of the tripolyphos- "ice phate at such surfaces is converted to pyrophosphate but are at a sufficiently high temperature to change the surfaces to a partially liquid state. The surfaces of the particles are thereby made viscid and the desired agglomeration occurs with much greater case than when all of the surface of the particles is liquefied. Furthermore, the density of the product is more readily controlled when the surfaces are partially liquefied. If the surface temperature of the particles is allowed to exceed the transition temperature at which all of the tripolyphosphate at the surfaces is converted to pyrophosphate, a rapid decomposition of the tripolyphosphate occurs and the cooled agglomerated particle has a substantially lower tripolyphosphate content than the original tripolyphosphate particles. If the surface temperatures are not high enough to change said surfaces to a partially liquefied state, no agglomeration of particles will occur thus enabling a variable rate of heating and variable feed stream temperatures up to the temperature at which the surfaces become viscid. The temperature of the surfaces of the hot particles can be controlled by measuring the bed temperature of the hot particles by a conventional thermocouple and adjusting the heat input according to the bed temperature. The term bed is the mass of particles of tripolyphosphate regardless of the method of applying heat or agitation. In a fluidized bed system, the bed temperature will be approximately the temperature of the hot gas used to fluidize the bed, while in a non-fluidized system, such as a rotary drum, the bed temperature will be the temperature of the surfaces of the hot particles. The viscid surfaced particles of tripolyphosphate are bound together by the viscid surfaces as they come into physical contact with each other. Therefore, to insure physical contact between the particles and subsequent desired agglomeration, agitation while the surfaces are viscid is necessary in the practice of this invention. After the desired agglomeration has occurred, it is necessary to cool the agglomerated particles so that the surfaces of the agglomerated particles are solidified to form a solid linkage before the entire particle has been converted to the partially liquefied state in order to prevent the formation of undesirably large chunks of alkali metal tripolyphosphate. As can be appreciated, certain additives useful as fiuxing agents can be used to lower the temperatures used in the practice of this invention and such use is considered to be Within the scope of this invention.

Although it is preferred in the practice of this invention to use sodiumtripolyphosphate, patassium tripolyphosphate, sodium tripolyphosphate and potassium tripolyphosphate mixtures and the sodium-potassium tripolyphosphate double salt, other alkali metal tripolyphosphate salts such as lithium, cesium, and rubidium tripolyphosphate salts and the like can be used. In addition other alkali metal phosphates can be substituted for a portion of the alkali metal tripolyphosphate and used to practice this invention provided that in the mixture so used the alkali metal tripolyphosphate is present in major amounts, that is, in amounts generally exceeding about by weight of the mixture.

In using sodium tripolyphosphate to practice this invention, it is necessary to heat a bed of finely divided sodium tripolyphosphate particles to form hot particles of sodium tripolyphosphate. The surfaces of said hot particles are at a temperature of from about 560 C to about 620 C. The bed temperature of the hot particles should be within the range of from about 560 C to about 620 C. in a non-fluidized system. At bed temperatures within this ran e, the surfaces of the particles will contain both liquid and solid and thereby be viscid to the special degree necessary to enable the desired agglomeration from contact with other hot particles. As can be appreciated, the bed temperature in a fluidized system will approach the temperature of the hot gas used to fiuidize the bed. In accordance with this invention, after the desired agglomeration has occurred, it is necessary to cool the agglomerated sodium tripolyphosphate particles below about 550 C. (at which temperature the surfaces become solid before the entire particle has been converted to the partially liquid state) to prevent formation of undesired large lumps of sodium tripolyphosphate.

In using potassium tripolyphosphate to practice this invention, it is necessary to heat a bed of finely divided potassium tripolyphosphate particles to form hot particles of potassim tripolyphosphate. The surfaces of said particles are from about 610 C. to about 40 C. At that surface temperature, the surfaces will contain both liquid and solid potassium tripolyphosphate and thereby be viscid, and the desired agglomeration will occur upon physical contact with other hot particles. To prevent the formation of undesired large lumps of tripolyphosphate, it is necessary after the desired agglomeration has occurred to cool the agglomerated particles of tripolyphosphate below 600 C. before the entire particle has been converted to the partially liquid state.

In the practice of this invention using mixtures of sodium tripolyphosphate and potassium tripolyphosphate, the surface temperature of the particles is from about 500 C. to about 620 C. When the particles of sodium and potassium tripolyphosphate come into physical contact with one another, the particles become agglomerated and upon cooling below about 500 (before the entire particle becomes partially liquid), the salts are bound together in the desired agglomerated form without excessive lumps. It should be noted that the presence of a small amount of potassium salt on a hot surface of sodium tripolyphosphate will lower the temperature at which the surfaces of the particles of sodium tripolyphosphate contain liquid without the danger of melting the whole particle. This property can be utilized advantageously to enhance particle agglomeration at a temperature lower than that used for the pure salt. Other salts, such as sodium sulfate, sodium acid sulfate, sodium acid pyrophosphate which lower the temperature at which the surfaces are partially liquefied can also be used.

In the practice of this invention using the sodiumpotassium tripolyphosphate double salt, it is necessary to heat the bed of finely divided particles to a temperature sulficiently high so that the surfaces of said particles are from about 500 C. to about 640 C. At that surface temperature, the surfaces will contain both liquid and solid and become viscid, enabling desired agglomeration upon physical contact with other particles. It is necessary to cool the agglomerated particles below about 500 C. before the entire particle has been converted to the partially liquefied state to prevent the formation of excessively large lumps.

An conventional means of heating the finely divided 1 alkali metal tripolyphosphate can be used to practice the invention. For example, direct heating by a conventional burner will give suitable results if the bed temperature is properly controlled. It is possible to supply heat by indirect heating, for example by use of an externally heated rotary drum, screw conveyor or vibrating conveyor. In addition, heating of the particles can be accomplished by a hot gas stream such as in a fluidized bed. A hot gas stream can also be used to pass through a bed of tripolyphosphate as in a rotary drum. A combination of heating methods can also be used, for instance, a vibrating conveyor can be directly heated by a hot gas stream and also indirectly heated by a heating medium in a jacket on the conveyor. It should be noted that the types of heating are given to more clearly explain the invention and are in no way intended to limit the scope of the invention.

Any conventional means of agitation can be used to cause particles to physically contact each other while the temperature of the particles is sufiiciently high to have a liquid-solid tripolyphosphate phase on the surfaces.

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For example, a rotary drum, in which the particles are tumbled together while the surfaces of the particles have a liquid-solid tripolyphosphate phase and are viscid, is a suitable type of continuous type of agitation.

Another form of agitation which can be used is a screw conveyor. For example, the alkali metal tripolyphosphate feed to the conveyor can be heated to the proper temperature, so as to have a liquid-solid tripolyphosphate phase on the surfaces, and the movement of the particles down the conveyor through the constant tumbling action will cause physical contact of the particles forming an agglomerate. The alkali metal tripolyphosphate can also be unheated prior to entering an indirectly heated conveyor in which heat is transmitted through the Walls of the conveyor to the tripolyphosphate. The particles upon reaching the proper temperature, whereby the surfaces are rendered viscid, will become agglomerated by the physical contact from the constant tumbling as the particles move through the conveyor.

An additional method of applying agitation to the finely divided particles is by using a fluidized bed in which hot gases. with sufficient heat contents to render the surfaces of the tripolyphosphate particles viscid, provides the necessary contact between the particles to cause the desired agglomeration. Similarly, hot gases of sufi'icient heat content to maintain the surfaces of a pre heated feed in the viscid state while providing the necessary agitation to insure physical contact can also be used.

Still an additional method of providing agitation is by means of a vibrating conveyor. The vibrating action of the conveyor will cause the particles to physically contact each other and form the agglomerated particles, bound together by the surfaces of the tripolyphosphate particles. A preheated feed stream, sufficiently hot so that the surface of the particles is viscid, can be supplied to the conveyor. Also, the necessary heating of an unheated feed stream can be done in or on the conveyor. It should be noted that the means of supplying agitation given herein are for exemplary purposes only and are not intended to limit the scope of the invention.

The finely divided particles can be agitated while being heated. The particles can also be heated and then agitated subsequent to the heating, but prior to being cooled to a temperature whereby the surfaces become solidified and are no longer viscid. Any means of enabling the solid particles to become bound together by the viscid, solidliquid phase existing on the surfaces of the finely divided particles can be employed. For example, the particles can be heated to a temperature sufficiently high whereby the surfaces are viscid (but are below the temperature where all of the alkali metal tripolyphosphate is changed to the alkali metal pyrophosphate), and after the particles are at that temperature, the particles can be agitated in such a manner so that the particles come into physical contact with one another and are bound together by the viscid surfaces of the particles. Also, the particles may be agglomerated by agitating to establish said contact while being heated. In any event, the particles must physically contact each other while the surfaces of the particles are in the above-described viscid, liquid-solid state so that the particles will be bound together by said viscid surfaces and form an agglomerate larger than the separate finely divided particles.

In the practice of this invention it is necessary to cool the agglomerated particles so that the surface of the agglomerated particles becomes solidified to form a solid linkage. In order to prevent the formation of undesirable large lumps in the hot bed, the cooling must occur before the nuclei of the hot particles of tripolyphosphate (which are joined together by the liquid on the surfaces to form the desired agglomerate) are heated to the temperature at which said nuclei become partially liquid.

Any conventional means of cooling the agglomerated particles can be used. For example, discharging out of the agitated zone into a zone cooled with currents of cold gases, such as atmospheric air is a suitable means of cooling. Another means to provide cooling is to discharge the agglomerated particles onto a conveyor in which the heat from the particles is transmitted to the conveyor and then from the conveyor to a suitable cooling medium. In addition, the conveyor can have currents of cold gas, such as atmospheric air, passing over the bed of agglomerated particles to provide the desired cooling. Cold gases, such as atmospheric air, can be used to cool the agglomerated particles while simultaneously conveying them to another location. An additional suitable means of cooling is to pass cold gases, such as atmospheric air, through a bed of agglomerated material in a rotary drum. Another suitable means of cooling is to discharge the hot agglomerated particles into a vibrating conveyor containing a sufficient amount of cold finely divided particles suitable as a feed stream for this invention. After separation into desired particle sizes, the feed stream particles are preheated and the agglomerated particles are sufficiently cooled. These methods of cooling are given for explanation purposes only and are not intended to limit the scope of the invention.

The time the particles can remain in the heating zone before the nuclei will become sufiiciently hot to convert to the partially liquid state will depend upon several variables; for example, the temperature of the entering feed stream, the initial particle size of the feed stream, the temperature of the heating medium, and the bed temperature of the heating zone. When using a feed stream at atmospheric temperature of about 25 C. and containing particles smaller than a U.S. Standard 100 mesh screen and using a fluidized bed with an inlet gas temperature of above about 620 C., it is generally preferred to retain the particles in the heating zone from about 0.1 minute to about 35 minutes. When using a direct flame from burning gas to heat a feed stream of the same temperature and particle size, it is preferred to retain the particles exposed to a direct flame for from about 3 seconds to about 60 seconds. When using hot gases heated to the temperature of about 700 C. and usin a rotary drum, it is preferred to have a retention time Within the heating zone from about 0.5 minute to about minutes. The retention time can be increased if the feed stream has a larger initial particle size because of the longer time required to transmit heat to the center of the particle thereby partially liquefying the entire particle. Similarly, the colder the initial temperature of the feed stream, the longer the particles can be exposed to the heating zone.

As can be appreciated, the three steps of (a) heating, (b) agitating, and (c) cooling can be conducted in the same piece of equipment. For example, a rotary drum can have a heating and agitating zone and a cooling zone and thereby discharge a cooled, desirably agglomerated particle of tripolyphosphate. Equally suitable would be to carry out the heating and agitating phases in one piece of equipment such as a vibrating conveyor, and cool in a separate piece of equipment, such as a pneumatic conveyor which would cool the agglomerated particles while conveying them to another location such as a storage bin.

Although it is preferred that at least 90% by weight of the tripolyphosphate feed to the heating section pass through a U.S. Standard 100 mesh screen and at least 70% by weight through a U.S. Standard 270 mesh screen. satisfactory results can be achieved with larger initial particle sizes. For example, a tripolyphosphate feed to a heating section with at least 60% by weight passing through a U.S. Standard 60 mesh screen will give a satisfactory product. Materials having even larger particle size can be used if desired.

It is to be noted that in the following examples the agglomerated particles have been separated into three fractions which have 'been named: oversize fraction, undersize fraction, and desired fraction. The oversize fraction is above a U.S. Standard 20 mesh screen. The undersize fraction passes through a U.S. 100 mesh screen, and the desired fraction has '3. particles size smaller than a U.S. Standard 20 mesh and larger than a U.S. Standard 100 mesh screen. The names of oversize, undersize, and desired fraction are given solely for clarity purposes in explaining the invention. All parts and percentages are by weight unless otherwise specified.

Example 1 A feed stock of finely divided sodium tripolyphosphate particles with a 91% sodium tripolyp'hosphate content with 90% passing through a U.S. Standard 100 mesh screen is fed into a rotary drum. The feed rate and drum rotation speed are adjusted to allow about 40 minutes of retention time within the rotary drum. Heat is supplied by a conventional burner using natural gas as a fuel. The atmosphere above the bed of sodium tripolyphosphate particles in the rotary drum is measured to be at about 620 C. at the feed end and about 550 C. at the discharge end. The bed temperature is allowed to rise to about 550 C. causing the surfaces of the particles to become viscid, that is, viscid-surfaced particles are formed. The tumbling action of the particles causes agglomeration to occur as the particles move from the feed end to the discharge end in the rotary drum. To prevent undesirably large lump formation, the particles are cooled to below about 540 C. within 10 minutes after discharge from the rotary drum. The cooled agglomerated particles are conveyed to a conventional screen to separate the particles into the three before-mentioned fractions. The desired fraction (20 to +100 mesh) is about 26% of the finely divided particle's charged to the drum. The oversize fraction (+20 mesh) is about 57% of the material charged. The undersize fraction (l00 mesh) is about 17% of the material charged.

The desired fraction (20 to +100 mesh) has a bulk density of about 0.5 gram per cc. The particles of the desired fraction are relatively hard and show no tendency to decrease to finer material. The desired fraction has a sodium tripolyphosphate content of 91%. grams of the granular desired fraction dissolves in 400 cc. of water in 3 minutes and forms a clear solution.

Example 2 A feed stock of finely divided sodium tripolyphosphate particles with a 91% of sodium tripolyphos-phate assay and through a U.S. Standard mesh screen is fed into the rotary drum. Feed rate and drum rotation speed are adjusted to allow about 30 minutes of retention time in the rotary drum. Heat is supplied by applying a flame from a conventional natural gas burner directly to a bed of tripolyphosp'hate in the rotary drum. The bed temperature is measured to be at about 600 C. at the point of flame impingement upon the bed. The tumbling action of the particles as they move through the rotary drum causes the agglomeration to occur. So that the desired agglomeration can occur without forming excessively large lumps, no heat is applied to the particles during the last half of the travel through the rotary drum. The flame is applied for about the first minute of travel. During the remaining time of travel, the bed temperature drops to about 560 C. and only agitation of the viscid particles occurs. The particles are discharged from the drum and cooled to below about 540 C. within 10 minutes. The particles are conveyed to a conventional screen to separate the particles into the three before-mentioned fractions. The desired fraction (-20 to +100 mesh) is about 35 of material charged. The oversize fraction (+20 mesh) is about 30% of material charged. The undersize fraction (+100 mesh) is about 35% of the material charged. The density of the desired fraction is about 0.9 gram per cc. The particles of the desired fraction have a sodium tripolyphosph-ate content of 91%. 50 grams of the granular desired fraction dissolves in 400 cc. of water in 3 minutes and forms a clear solution.

Example 3 In a continuous process a feed stock of l mesh sodium tr-ipolyphosphate with a sodium tripolyphosphate content of 91% is fed to a rotary drum heated from a conventional natural gas or oil burner. The heat is supplied by the hot combustion gases over the bed in the rotary drum. The feed rate and the rate of revolution of the drum are adjusted so that particles pass through the drum in about 40 minutes. The bed temperature for about the last 20 minutes of the passage through the rotary drum is maintained about 560 C. The atmosphere above the bed at the feed end of the rotary drum is about 700 C. and falls to about 650 C. at the discharge end. During the first 10 minutes of passage through the rotary drum with a bed temperature of about 560, the particles become viscid and start to agglomerate. By holding the bed temperature in the last 30 minutes above about 550 n C., the particles continue to agglomerate to form the desired size. The agglomerated particles then discharge into a vibrating conveyor which has atmospheric air passing over the bed on the conveyor. The volume of air flow is controlled so that the bed temperature is below about 550 C. within the first 5 minutes of travel on the conveyor thus preventing further agglomeration. The agglomerated product is passed through a roller mill to break large granules before it is sized by conventional screens with openings approximating a US. Standard 20 and 100 mesh. The desirable fraction (-20 to 100 mesh) is withdrawn and about 70% of the material is fed to the rotary drum. The sodium tripoly'phosphate content of the desired fraction is about 91%. The small amount of oversize fraction (+20 mesh) is milled to l00 mesh screen size and combined with the undersize {-100 mesh) to form a recycle stream representing about 30% of the feed to the rotary drum. The recycle stream is combined with new finely divided sodium tripolyphosphate to give the proper feed rate to the rotary drum.

Example 4 The feed stock of finely divided sodium tripolyphosphate particles with a 91% sodium tripolyphosphate content and 90% passing through a US. Standard 100 mesh screen is fed into a fluidized bed. I-Iot combustion gases, heated to about 650 C., keep the bed fluid. The hot gases cause the particles to have repeated contact with each other as they pass through the bed. The bed temperature of the fluidized bed measures about 620 C., approaching the temperature of the hot gas which is used not only to fluidize the bed, but also to cause the agitation, and to provide suflicient heat to render the surfaces of the particles viscid. The agglomerated particles are cooled to below about 550 C. after discharge from the fluidized The feed stock of finely divided mixture of 90% sodium tripolyphosphate particles and potassium tripolyphosphate and with 90% through a U.S. Standard 100 mesh screen is fed into a closed vibrating conveyor. Heat is supplied by hot gases from a conventional burner using natural gas as a fuel. The atmosphere above the bed of the tripolyphosphate mixture is about 640 at the feed end and about 560 C. at the discharge end. The bed temperature reaches a maximum of about 560 C.,

causing particle surfaces to become viscid. The tumbling action of the particles as they pass down the vibrating conveyor causes the desired agglomeration to occur. The particles are discharged from the conveyor into a second vibrating conveyor which conveys them to a conventional screen. In the second conveyor, atmospheric air, drawn countercurrent to the flow of particles on the conveyor, causes the particles to be cooled to below about 500 C. within 8 minutes of travel. The particles are separated into three before-mentioned fractions by a conventional screen. The desired fraction to +100 mesh) is about of the finely divided particles charged to the conveyor. The oversize fraction (+20 mesh) is about of material charged and the undersize fraction (l00 mesh) is about 30% of the material charged. The desired fraction (20 to +100 mesh) has a bulk density about 0.6 gram per cc. The particles of the desired fraction are relatively hard and show no tendency to degrade to finer material.

Example 6 .A feed stock comprising of sodium tripolyphosphate and through a US. Standard mesh screen and 50% of potassium tripolyphosphate and with 90% through a 100 mesh screen is fed into a rotary drum heated by a conventional burner. The atmosphere above the mixture of sodium and postassium tripolyphosphate particles is about 700 C. at the feed end and about 650 C. at the discharge end. The bed temperature is allowed to rise to 560 C. but not allowed to exceed 620 C. The tumbling action of the particles as they pass through the rotary drum cause the particles to agglomerate as they contact each other. The feed rate and drum rotation speed are adjusted to allow about 40 minutes of retention time within the rotary drum. The particles are cooled to below about 540 C. Within 10 minutes after discharge from the rotary drum. After being conveyed to a conventional screen, the panticles are separated into three before- .rnentioned fractions.

The desired fraction (20 to +100 mesh) is about 30% of finely divided particles charged to the drum. The oversize fraction (+20 mesh) is about 30% of the material charged and the undersize fraction is about 40% of the material charged. The desired fraction (20 to +100 mesh) has a bulk density of about 0.5 gram per cc. The particles of the desired fraction are relatively hard and show no tendency to degrade to finer material.

Example 7 A feed stock comprising the sodium potassium tripolyphosphate double salt with an assay of 90% of the beforementioned tripolyphosphate salts and a screen size of 90% through a 100 mesh screen is fed to a fluidized bed. The bed is kept in a fluid condition by hot combustion gases of about 650 C. supplied by a conventional burner using natural gas as a fuel. The bed temperature in the fluidized bed is about 630 C. At that temperature the surfaces of the particles will become viscid and the constant tumbling within the fluidized bed causes the contact necessary to enable the particles with their viscid surfaces to agglomerate. The retention time in the fluidized bed is about 40 minutes. The material is cooled to below about 540 C. in 10 minutes after discharge from the fluidized bed and is subsequently conveyed to a conventional screen where the particles are separated into three before-mentioned fractions. The percent by weight of the three fractions is as follows:

The desired fraction has a bulk density of about 0.5 gram per cc. The assay of the desired fraction is equiv- 9 alent to that of the feed stock with a 90% tripolyphosphate salt content.

What is claimed is:

1. A process for agglomerating finely divided particles of alkali metal tripolyphosphate which comprises (a) making viscid-surfaced particles by heating said particles in an atmosphere having a temperature sufficiently high to change the surfaces of said particles to a partially liquid state rendering said surfaces viscid but below the transition temperature at which all of the alkali metal tripolyphosphate at the surfaces of said particles is converted to alkali metal pyrophosphate; (b) agitating said viscidsurfaced particles to thereby produce agglomerated particles of alkali metal tripolyphosphate; and (c) cooling said agglomerated particles of tripolyphosphate to a temperature at which the surfaces of the agglomerated particles are solidified but before the solid alkali metal tripolyphosphate nuclei of said viscid-surfaced particles forming said agglomerated particles become partially liquefied.

2. The process of claim 1 wherein the alkali metal tripolyphosphate is a member selected from the group consisting of odium tripolyphosphate, potassium tripolyphosphate, mixtures of sodium tripolyphosphate and potassium tripolyphosphate and the sodium potassium tripolyphosphate double salt.

3. The process of claim 1 wherein the alkali metal is sodium.

4. The process of claim 1 wherein the alkali metal is potassium.

5. The process of claim 1 wherein the alkali metal tripolyphosphate is a mixture of sodium tripolyphosphate and potassium tripolyphosphate.

6. The process of claim 1 wherein the alkali metal tripolyphosphate is the sodium potassium tripolyphosphate double salt.

7. A process for agglomerating finely divided particles of sodium tripolyphosphate which comprises (a) making viscid-surfaced particles of sodium tripolyphosphate by heating said particles having a particle size of smaller than a US. Standard 60 mesh screen, for from about 0.05 minute to about 40 minutes in an atmosphere having a temperature sufficiently high to have the surface temperature of said viscid-surfaced particles above about 550 C. but below about 620 C. thereby rendering the surfaces of said particles viscid and in the partially liquid state; (b) agitating said viscid-surfaced particles to thereby produce agglomerated particles of sodium tripolyphosphate; and (c) cooling said agglomerated particles to below about 550 C. at which the surfaces of the agglomerated particles are solidified but before the solid sodium tripolyphosphate nuclei of said viscid-surfaced particles forming said agglomerated particles become partially liquefied.

8. A process for agglomerating finely divided particles of potassium tripolyphosphate which comprise (a) making viscid-surfaced particles of potassium tripolyphosphate by heating said particles having a particle size of smaller than a US. Standard 60 mesh screen, for from about 0.5 minute to about 40 minutes in an atmosphere having a temperature suficiently high to have the surface temperature of said viscid-surfaced particles above about 610 C. but below about 640 C. thereby rendering the surfaces of said particles viscid and in the partially liquid state; (b) agitating said viscid-surfaced particles to thereby produce agglomerated particles of potassium tripolyphosphate; and (c) cooling said agglomerated particles to below about 610 C. at which the surfaces of the agglomerated particles are solidified but before the solid potassium tripolyphosphate nuclei of said viscid-surfaced particles forming said agglomerated particles become partially liquefied.

9. A process for agglomerating finely divided particles of sodium tripolyphosphate and potassium tripolyphosphate which comprises (21) making viscid surfaced particles of sodium tripolyphosphate by heating said particles in an atmosphere having a temperature sufficiently high to have the surface temperature of said viscid-surfaced particles above about 500 C. but below about 640 C. thereby rendering the surfaces of said viscid-surfaced particles viscid and in the partially liquid state; (b) agitating said viscid-surfaced particles to thereby produce agglomerated particles of sodium and potassium tripolyphosphate; and (c) cooling said agglomerated particles to below about 500 C. at which the surfaces of the agglomerated particles are solidified but before the solid tripolyphosphate nuclei of said viscid-surfaced particles forming said agglomerated particles become partially liquefied.

10. A process for agglomerating finely divided particles of sodium tripolyphosphate which comprises (a) making viscid-surfaced particles of sodium tripolyphosphate by heating sodium tripolyphosphate particles, for from about 0.05 minute to about 10 minutes; said particles having 21 particles size about by weight through a US Standard mesh screen and about 30% by weight retained on a U5. Standard 270 mesh screen; in an atmosphere having a temperature sufficiently high to have the surface temperature of said viscid-surfaced particles about about 550 C. but below about 600 C. thereby rendering the surfaces of said hot particles viscid and in the partially liquid state; (b) agitating said viscid-surfaced particles to thereby produce agglomerated particles of sodium tripolyphosphate; and (c) cooling said agglomerated particles to below about 550 C. within about 10 minutes after the bed temperature of said particles reaches 550 C.; whereby the surfaces of the agglomerated particles are solidified before the solid sodium tripolyphosphate nuclei of said viscid-surfaced particles forming said agglomerated particles become partially liquefied.

11. A recycle process for agglomerating finely divided alkali metal tripolyphosphate which comprises (a) making viscid-surfaced particles by heating said particles in an atmosphere having a temperature sufficiently high to change the surfaces of said particles to a partially liquid state rendering said surfaces viscid but below the transition temperature at which all of the alkali metal tripolyphosphate at the surfaces of said particles is converted to alkali metal pyrophosphate; (b) agitating said viscid-surfaced particles to thereby produce agglomerated particles of alkali metal tripolyphosphate; (c) cooling said agglomerated particles of tripolyphosphate to a temperature at which the surfaces of the agglomerated particles are solidified but before the solid alkali mettal tripolyphosphate nuclei of said viscid-surfaced particles forming said agglomerated particles becomes partially liquefied; (d) separating the agglomerated alkali metal tripolyphosphate into at least two fractions based upon particle size distribution, at least one fraction consisting of particles exceeding a predetermined particle size and at least one fraction consisting of particles having a smaller particle size; and (e) returning at least one fraction consisting of particles having a smaller particle size to said heated zone.

12. A recycle process for agglomerating finely divided sodium tripolyphosphate which comprises (21) making viscid-surfaced particles of sodium tripolyphosphate by heating said particles in an atmosphere having a temperature sufficiently high to have the surface temperature of said viscid-surfaced particles above about 550 C. but below about 620 C. thereby rendering the surfaces of said particles viscid and in the partially liquid state; (b) agitating said viscid-surfaced particles to thereby produce agglomerated particles of sodium tripolyphosphate; (c) cooling said agglomerated particles to below about 550 C. at which the surfaces of the agglomerated particles are solidified but before the solid sodium tripolyphosphate nuclei of said viscid-surfaced particles forming said agglomerated particles become partially liquefied; (d) separating the agglomerated alkali metal tripolyphosphate into at least two fractions based upon particle size distribution, at least one fraction consisting of particles exceeding a predetermined particle size and at least one fraction consisting of particles having a smaller particle 11 size; and (e) returning at least one fraction consisting of particles having a smaller particle size to said heated zone.

13. A continuous process for agglomerating finely divided alkali metal tripolyphosphate which comprises a) making viscid-surfaced particles by heating said particles in an atmosphere having a temperature sufficiently high to change the surfaces of said particles to a partially liquid state rendering said surfaces viscid but below the transition temperature at which all of the alkali metal tripolyphosphate at the surfaces of said particles is converted to alkali metal pyrophosphate; (b) agitating said viscid-surfaced particles to thereby produce agglomerated particles of alkali metal tripolyphosphate; (c) cooling said agglomerated particles of tripolyphosphate to a temperature at which the surfaces of the agglomerated particles are solidified but before the solid alkali metal tripolyphosphate nuclei of said viscid-surfaced particles forming said agglomerated particles becomes partially liquefied; (d) separating the agglomerated alkali metal tripolyphosphate into at least two fractions based upon particle size distribution, at least one fraction consisting of particles exceeding a predetermined particle size and at least one fraction consisting of particles having a smaller particle size; and (e) returning at least one fraction consisting of particles having a smaller particle size to said heated zone along with a feed stream of finely divided alkali metal tripolyphosphate particles.

14. A continuous process for agglomerating finely divided sodium tripolyphosphate which comprises ta) making viscid-surfaced particles of sodium Iripolyphosphate by heating said particles in an atmosphere having a temperature sufficiently high to have the surface temperature of said viscid-surfaced particles above about 550 C. but below about 620 C. thereby rendering the surfaces of said particles viscid and in the partially liquid state; (b) agitating said viscid-surfaced particles to thereby produce agglomerated particles of sodium tripolyphosphate; (c) cooling said agglomerated particles to below about 550 C. at which the surfaces of the agglomerated particles are solidified but before the solid sodium tripolyphosphate nuclei of said viscid-surfaced particles forming said agglomerated particles become partially liquefied; (d) separating the agglomerated particles of sodium tripolyphosphate at least two fractions whereby the first fraction consists of particles larger than a U.S. Standard mesh screen and whereby the second fraction consists of particles smaller than a US. Standard 100 mesh screen; (e) withdrawing the fraction larger than a US. Standard 100 mesh screen; and (f) returning the fraction smaller than a US. Standard 100 ,mesh screen to said heated zone.

References Cited UNITED STATES PATENTS 986,271 3/1911 Dicke 23313 2,035,845 3/1936 Stanton 233 13 3,160,472 12/1964 Metcalf 23106 OSCAR R. VERTIZ, Primary Examiner.

H. S. MILLER, Assistant Examiner. 

